1. Field of the Invention
The present invention relates generally to methods and apparatus for calibrating a remote system which employs coherent signals and, more particularly, to a method and apparatus for calibrating a remote phased array system.
2. Prior Art
Active phased array systems or smart antenna systems have the capability for performing programmable changes in the complex gain (amplitude and phase) of the elemental signals that are transmitted and/or received by each respective element of the phased array system to accommodate different beam-forming scenarios. Communications satellites equipped with phased array systems are desirable since satellites so equipped have an intrinsic performance advantage over satellites with conventional reflector antennas. For example, a communications satellite with a phased array system can offer the following advantages: reconfigurable beam patterns ranging from broad-uniform continental coverage down to narrow spot beam patterns with 3 dB widths of about 1 degree; flexibility in varying the level of effective isotropic radiated power (EIRP) in multiple communication channels; and means for providing graceful system performance degradation to compensate for component failures. As conditions for the phased array system in the satellite can change in an unpredictable manner, regularly scheduled calibration for characteristics of the system, such as phase and amplitude characteristics, is generally required to assure optimal system performance.
In order to obtain meaningful estimates of the respective complex gains for the elemental signals respectively formed in each element of the phased array system, the calibration process must be performed in a time window that is sufficiently short so that the complex gains for the respective elemental signals transmitted from each element are substantially quasi-stationary. For a typical geostationary satellite application, the relevant time windows are dominated by two temporally variable effects: changes in the transmitted elemental signals due to variable atmospheric conditions encountered when such signals propagate toward a suitable control station located on Earth; and changes in the relative phase of the transmitted elemental signals due to thermally induced effects in the satellite, such as phase offsets in the respective circuit components for each respective element of the phased array system, and physical warpage of a panel structure employed for supporting the phased array. The thermally induced effects are caused primarily by diurnal variations of the solar irradiance on the phased array panel.
Most calibration techniques of the prior art are essentially variations on the theme of individually measuring, one at a time, the respective complex gain of each single element (SE) of the phased array system while all the other elements of the phased array system are turned off. Although these calibration techniques (herein referred as SE calibration techniques) are conceptually simple, these SE calibration techniques unfortunately have some fundamental problems that make their usefulness questionable for meeting the calibration requirements of typical phased array systems for communications satellites. One problem is the difficulty of implementing a multipole microwave switching device coupled at the front end of the respective electrical paths for each elemental signal so as to direct or route suitable test signals to any single element undergoing calibration. This multipole switching device is typically necessary in the SE calibration techniques to measure the complex gain for the elemental signal respectively formed in any individual element undergoing calibration at any given time. Another problem of the SE calibration techniques is their relatively low signal-to-noise ratio (SNR). This effectively translates into relatively long measurement integration times. At practical satellite power levels, the integration times required to extract the calibration measurements for the SE calibration techniques are often too long to satisfy the quasi-stationarity time window criteria described above. In principle, one could increase the effective SNR of the SE process by increasing the power of the calibration signals transmitted from each element. However, as each element of the phased array system is usually designed to operate at near maximum power, as dictated by the power-handling capacity and linearity constraints for the circuit components in each element, it follows that arbitrary additional increases in power levels are typically not feasible. Thus it is desirable to provide a calibration method that allows for overcoming the problems associated with SE calibration techniques.
Furthermore, U.S. Pat. No. 5,572,219 to Silverstein et al. (hereinafter xe2x80x9cSilversteinxe2x80x9d), incorporated herein by its reference, discloses methods and apparatus for remotely calibrating a system having a plurality of N elements, such as a phased array system. The method of Silverstein includes generating coherent signals, such as a calibration signal and a reference signal having a predetermined spectral relationship between one another. The calibration signal which is applied to each respective one of the plurality of N elements can be orthogonally encoded based on the entries of a predetermined invertible encoding matrix, such as a binary Hadamard matrix, to generate first and second sets of orthogonally encoded signals. The first and second sets of orthogonally encoded signals and the reference signal are transmitted to a remote location. The transmitted first and second sets of orthogonally encoded signals are coherently detected at the remote location. The coherently detected first and second sets of orthogonally encoded signals are then decoded using the inverse of the predetermined invertible encoding matrix to generate a set of decoded signals. The set of decoded signals is then processed for generating calibration data for each element of the system. Although a significant improvement over the SE calibration techniques of the prior art, the controlled circuit encoding (CCE) techniques of Silverstein can be improved, resulting in a reduced calibration time and lower processing burden.
Therefore it is an object of the present invention to provide a method and apparatus for calibrating a remote system which remotely estimates the electrical parameters of remote system such as a phased antenna arrayxe2x80x94namely the complex gain of the quiescent channels and component (discrete) beamforming elements.
It is a further object of the present invention to provide a method and apparatus for calibrating a remote system which requires minimal additional dedicated hardware.
It is yet a further object of the present invention to provide a method and apparatus for calibrating a remote system, such as a phased antenna array and operate within a sufficiently small interval in time to prove valid for arrays operating in a dynamic environment.
The present invention is directed to a method for calibrating remote phased antenna arrays such as those existing on satellite platforms. The method provides estimates of the electrical parameters of an phased antenna array by encoding the elemental signals via selectable devices within the array itself. As a result, the calibration system is composed largely of existing hardware.
The methods of the present invention are a variant of the Silverstein method in which so-termed xe2x80x9cForwardxe2x80x9d and xe2x80x9cReversexe2x80x9d measurement sets were employed in the calibration process. The methods of the present invention require either of the two data sets. The principal advantage of the methods of the present invention is that the total calibration time may be reduced due to the absence of approximately half the number of device settling interval naturally existing between measurement dwells. The total measurement time, however, remains constant to provide the same parameter estimation accuracy.
Accordingly, a method for calibrating a remote system having a plurality of N elements is provided where N is a positive integer number, an input signal to each of said N elements is processed according to beamforming bits to determine the output of a corresponding element, and the output of the plurality of N elements is a composite signal. The method comprises the steps of: (a) transmitting a calibration signal to input the plurality of N elements of the remote system; (b) selecting a first set of beamforming bits for each of the plurality of N elements based upon entries of a predetermined invertible matrix; (c) processing the calibration signal at the remote system according to the beamforming bits for each of the N plurality of elements; (d) detecting a reference signal from the remote system and the composite signal transmitted from the N plurality of elements based on the first set of beamforming bits; (e) repeating steps (b)-(d) for successive sets of beamforming bits to generate a set of signals; and (f) processing the set of signals for generating calibration data for each of the N plurality of elements of the remote system.